xzgrep: use `grep -E/-F` instead of `egrep` and `fgrep`

`egrep` and `fgrep` have been deprecated in GNU grep since 2007, and in
current post 3.7 Git they have been made to emit obsolescence warnings:
https://git.savannah.gnu.org/cgit/grep.git/commit/?id=a9515624709865d480e3142fd959bccd1c9372d1

src/scripts/xzgrep.1

@@ -67,9 +67,9 @@ is invoked as
 or
 .B xzfgrep
 then
-.BR egrep (1)
+.B grep \-E
 or
-.BR fgrep (1)
+.B grep \-F
 is used instead of
 .BR grep (1).
 The same applies to names
@@ -87,9 +87,9 @@ environment variable is set,
 .B xzgrep
 uses it instead of
 .BR grep (1),
-.BR egrep (1),
+.BR "grep \-E" ,
 or
-.BR fgrep (1).
+.BR "grep \-F" .
 .SH "SEE ALSO"
 .BR grep (1),
 .BR xz (1),

src/scripts/xzgrep.in

@@ -29,8 +29,8 @@ xz='@xz@ --format=auto'
 unset GZIP BZIP BZIP2 LZOP
 
 case ${0##*/} in
-  *egrep*) prog=xzegrep; grep=${GREP:-egrep};;
-  *fgrep*) prog=xzfgrep; grep=${GREP:-fgrep};;
+  *egrep*) prog=xzegrep; grep=${GREP:-grep -E};;
+  *fgrep*) prog=xzfgrep; grep=${GREP:-grep -F};;
   *)       prog=xzgrep; grep=${GREP:-grep};;
 esac